Passive monopulse ranging system

ABSTRACT

A monopulse ranging system where incoming signals from two antennas are converted to an intermediate frequency and passed through corrective delays with one of the delays being controlled by a delay balance discriminator. The detected envelopes of the signals are fed to a differential amplifier, the output thereof being fed to a range computer and the delay balance discriminator detects the differences to be fed back to one of the delay controls.

United States Patent 3 Claims 4 Drawing Figs the delays bein conuolled ba dela balance discriminator.

g Y y [52] US. Cl I 7 343/112 The detected envelopes of the signals are.fed to a drfierential. [51] IiitI'CI'M' G01s 5/14, amplifier, the outputthereof being fed to a range computer GOls l 1/00 and the delay balancediscriminator detects the differences to [50] field of Search 343/1 12.3be fed back to one of the delay controls.

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Leonard P. Balazer, Sunnyvale, Calif.

Appl. No. 817,616

Filed Apr. 16, 1969 Patented May 4, 197 l Assignee The United States ofAmerica as represented by the Secretary of the Air Force PASSIVEMONOPULSE RANGING SYSTEM [56] References Cited UNITED STATES PATENTS3,134,104 5/1964 Murphree et al. 343/1 l2(.3) 3,430,243 2/1969 Evans343/1 l2(.3)

Primary Examiner-Rodney D. Bennett, Jr. Assistant Examiner-Richard E.Berger Att0rneysHa.rry A. Herbert, Jr. and Julian L. Siegel ABSTRACT: Amonopulse ranging system where incoming signals from two antennas areconverted to an intermediate frequency and passed through correctivedelays with one of PATENTED MAY 41911 SHEET 1 [IF 2 xs kv 3 hm h b Q Q Q.n NJ/l} H {is l l TEE INVENTORJ.

QYBEV PASSIVE MONOPULSE RANGING SYSTEM BACKGROUND OF THE INVENTION Thisinvention relates to range detectors and more particularly to a passivemonopulse-ranging system for determining the distance between a vehicleand any electromagnetic radiating source.

Previous passive ranging techniques require more than one data signalsample, such as directional finding together with triangulation, andover a long time period to calculate the range. Some of these techniquesare known as vertical triangulation, horizontal triangulation, squintangle, and azimuth/elevation ranging.

This invention makes it possible to passively locate an electromagneticpulse emitter on the basis of one received pulse. It thus makes athree-dimensional monopulse system practical. Ranging measurements areinstantaneous and vehicle perturbations do not directly afiect rangingaccuracy.

SUMMARY OF THE INVENTION The monopulse-ranging system of this inventioncan be used to determine the distance or range from a sensing platformto any electromagnetically radiating emitter, and presents a specificutilization of the monopulse principle in an aircraft to ground radarranging problem. The sensing platform can be a moving aircraft with oneantenna located at the nose and the other at the tail and the emittercan be a ground based pulsed radar.

It is therefore an object of the invention to provide a system fordetermining the distance between a vehicle and a source ofelectromagnetic radiation.

It is another object to provide a system for passively locating anelectromagnetic pulse emitter on the basis of one received pulse.

It is still another object to provide a ranging measurement system whichis instantaneous and in which vehicle perturbation does not affectranging accuracy.

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiment in the accompanying drawings, wherein:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing thesignal-to-noise ratio plotted against range for various angles to theemitter.

FIG. 2 is a diagram of a vehicle showing the angular variation;

FIG. 3 is a block diagram showing the monopulse-ranging system; and

FIG. 4 shows the output waveforms of the differential amplifier.

DETAILED DESCRIPI'IONOF THE PREFERRED EMBODIMENT The present inventionenables the operator of a device that can be located in an airbornevehicle to determine the range of the vehicle to any electromagneticemitter within its detection range. Rangingis done passively and can beaccomplished on the reception of a single pulse, hence, monopul'seranging.

The power received by an antenna from a transmitter is expressed by theone-way radar range equation A 2 P =P GTG where P Power received P Powertransmitted G Receiver antenna gain, max. G Transmitters antenna gain,max. A Wavelength R Range.

All of the above parameters except R and P can be assumed to remainconstant during the short period of a range measurement; therefore,received power can be related to R y K where A 2 K=P G G The firstderivative of received power relative to range can be obtained bydifferentiating the above expression for P to obtain This quantity couldbe obtained by comparing a single radar pulse received at two locationswhich are a known radial distance apart. By dividing the total powerreceived by one antenna by the first derivative, the measurement can benormalized and made independent of K.

The negative sign may be eliminated by considering R=R -R to benegative. Accordingly,

AR To solve this equation it is not necessary to know anything about thetransmitter characteristics. However, AP is a small quantity and thermalreceiver noise and the receiver noise factor are significant in relationto AP.

Receiver noise and multipath propagation error are possible sources oferror in this method of ranging. Multipath error effects will vary withrange and terrain irregularity while receiver noise can be consideredconstant. Receiving system absolute calibration error is largelycanceled since AP R and P can be measured simultaneously by much thesame equipment. Calibration error cancelation can be illustrated byrewriting the first derivative range equation with error factors andassuming P and AP measurements are made by the same equipment. Thecalibration error factor is designated as Ep, such that measuredpowerequals actual power times l-l-E and P are-very nearly the same level,receiver linearity errors are negligible. P is more correctly urlm 2however, the difference between P and P or m amounts to one part in4,000, and consequently the error due to using P or P for P isnegligible.

To indicate the ranging accuracy which can be achieved by thisinvention, signal-to-noise ratio can be calculated for a worst casesearch radar; as an example, one located km. from a ranging aircraftwith the receiving antennas set 25 meters apart.

It is assumed that the. radar transmits 750 kw, peak power, G is 40 db.,G is 6 db. and )t is 0.107 m., corresponding to 2.8 GHz. Then, K=2.16X10 W-M.

Signal-to-noise. voltage ratio, AV IV in a system incorporating linearsignal detection, is calculated by solving a signal voltage rangeequation derived from the power range equation. To derive this equation,consider that where V is receiver signal voltage. Since P =K/R thenDifferentiating V with respect to R,

Thermal noise voltage V); at an operating temperature of 290 K, MHzbandwidth and receiver noise factor, F=3.l6, is 5.02 l0 V. This valuecannot be used directly since when two uncorrelated random noise signalsare compared, the total r.m.s. noise voltage, V is calculated by whereV, and V are the noise voltages of the separate signals.

VN! N2, NT= V 2VN1 Therefore, difference noise voltage is V =7.1 10volts, and single pulse voltage signal to noise is This is the ratio ofthe difference signal voltage to total noise voltage.

In the operational situation an aircraft will always receive a pulsetrain rather than a single pulse. The least number of pulses in a pulsetrain can be calculated by considering pulse rate, scan rate, and beamwidth of a worst case search radar with a beam width of 1, a 3 rpm. scanrate, and a pulse rate of 360 pps. The number of pulses, N, interceptedby the aircraft would be 1 sec./min. 360 pulses N 3/min. sec 20 pulsesthe power signal to noise improvement, G that can be gained by pulseintegration is G =NA where A is 0.8 for 20 pulses.

Since voltage signal to noise is equal to the square root of powersignal to noise, voltage signal to noise gain, G is 60 o= /oF-N-=20-*=3.3 1=10.4 db.

Therefore, the integrated voltage signal to noise at 100 km. is

S/N,-,=3.31X5.l7=17.l=24.7 db.

Fig. 1 shows range in relation to signal to noise. This information isgiven for four different aircraft bearing angles, 0 to emitterdirection. Since the difference signal strength is approximatelyproportional to the difference in radial distance of the two antennasfrom the transmitter, signal strength is proportional to cos 0 anddecreases as the aircraft heading deviates from the emitter bearing asshown in FIG. 2 where 6 is the angle of the vehicle 11 in relation tothe target.

The operation of the invention is explained referring to the blockdiagram of FIG. 3 which shows two incoming signals and two channels.Channel A is derived from the nose of the vehicle and Channel B from thetail of the vehicle. Incoming signals from antennas 13 and 15 first passthrough RF gates 14 and 16 and directional couplers 18 and 20 and thento RF amplifiers 17 and 19. After amplification the signals are mixed inmixers 21 and 23 with the signal from local oscillator 25 to produceintermediate frequency signals. The frequency of the local oscillatorsignal is locked by an AFC link control 26 such as the targetidentification and acquisition (TIA) unit which is independent of theranging system. The ranging frequency is adjusted simultaneously by thesame means and is fed to calibration generator 47. The incoming signalchannel is also gated by a TIA supplied gating signal from control 30 togate generator 32 in order to pass only the pulse train of interestthrough RF amplifiers 17 and 19. The TIA supplied signals enable theranging system to select one emitter in an interleaved pulse train.Without the TIA or similar control, this capability would have to besupplied with additional equipment. After leaving mixers 21 and 23, IFsignals are amplified by IF amplifiers 27 and 29 through time delaynetworks 31 and 3. This delay compensates for the time required by theincident wave to traverse the distance in radial difference between thetwo antennas. In an aircraft installation, this time will beproportional to the aircraft heading relative to the emitter directionand will vary according to D=D cos 6.

Where D is the delay time, D is the maximum delay when the angle, 6between the aircraft heading and target direction is zero. The channel Btime delay is fixed, but adjustable during ground alignment andcheckout. The channel A time delay is variable and the delay time isdetermined by a DC voltage level from delay balance discriminator 35.The maximum differential delay corresponds to the time for anelectromagnetic wave to traverse the distance between antennas or 83 ns,corresponding to 25 m. in this case of the example used.

Since the ranging system is designed to operate at all headings relativeto target direction, channel A delay must vary over a range from 83 ns.less than channel B to 83 ns. more than channel B. To accomplish this,channel B is set at a nominal ns. at checkout and alignment, and channelA varies from zero to 200 ns. After passing through the delay net works,the incoming signals are envelope detected by detectors 37 and 39 andfed into differential amplifier 41.

If the two differential amplifier input signals are not synchronous,large amplitude outputs will occur during the time in which the signalsdo not overlap, as shown in FIG. 4. If the channel A signal leads thechannel B signal, the difference signal will be equal to the value ofthe channel A signal alone until the channel B signal arrives. This willcause a high amplitude positive excursion at the beginning of the outputpulse waveform. When both signals are present, the output will consistof a relatively low amplitude difference signal and this will befollowed by a high amplitude negative excursion when the channel A pulsehas passed through the amplifier and the channel B signal is stillpresent. If the channel B signal led the channel A signal, this sequencewould be reversed and the output waveform would consist of a highamplitude negative excursion followed by the difference signal and thenby a high amplitude positive excursion.

The presence and sequence of these high amplitude excursions at thebeginning and end of the difference pulse waveform is detected by delaybalance discriminator 35. A proportional control voltage is generatedand fed back to variable time delay 32, to automatically equalize thetime of appearance of the two signals at the output of the respectivechannel. The polarity of the control voltage determines the sense of thefeedback signal and minimizes the output of differential amplifier 41.This reduces the high amplitude excursions to no more than a spike atthe beginning and end of the pulse waveform. I

The signal from differential amplifier '41 is next passed through lowpass filter 43 to eliminate the spikes due to residual delay imbalanceand then fed to range computer 45.

An example of a range computer that can be used in the present inventionis disclose in our copending application, filed herewith.

The channel B configuration is the same as channel A with the exceptionthat the delay is fixed except during alignment and the channel gain iscontrolled by feedback from range computer 45 to RF amplifier 19.

lnflight calibration of the monopulse ranging system balances the gainresponse of both channels. RF signals of equal amplitude are insertedinto both signal channels, channel A from automatic gain control 28 andchannel B from range computer 45. Channel B is automatically adjusted toobtain a null difference signal output. The calibration signal is cw sothat time delays are not involved. The system is calibrated before eachrange measurement as soon as the transmitted frequency is determined andgated by the TlA equipment or other control and the receivers andcalibration generator 47 are timed to this frequency. The calibrationsignal is inserted into directional couplers 18 and 20 at the receiverinput at a level equal to a signal level while incoming signals areexcluded by input gates 14 and 16. During calibration and duringrangemeasurement the gain of channel A is clamped at the level reached whilereceiving the input signal during the TlA cycle The gain of channel B isnext adjusted to obtain a null in the difference signal input into rangecomputer 45 while the calibration signal is inserted into the system.Both channel gains remain set at this level during the range measurementof the particular emitter. This process is repeated before each rangedetermination. The range can be read out by display 49. i

We claim:

1. A monopulse-ranging system comprising:

a. first and second means spaced a predetermined distance for receivinga signal; b. a local oscillator,

c. a first mixer fed by the local oscillator and the firstsignalreceiving means;

d. a variable time delay fed by the first mixer;

e. a first envelope detector fed by the variable time delay;

f. a second mixer fed by the local oscillator and the secondsignal-receiving means;

g. a second time delay fed by the second mixer;

h. a second envelope detector;

i. a differential amplifier fed by the first and second envelopedetectors;

j. a delay balance discriminator fed by the differential arnplifier andconnected to the variable delay for the control thereof;

k. a low pass filter fed by the differential amplifier; and

1. means for computing the range fed by the output of the low passfilter, a reference signal from the delay balance discriminator, and thesecond envelope detector.

2. A monopulse-ranging system according to claim 1 which furthercomprises:

a. first and second gated amplifiers interposed between the first andsecond signal-receiving means and the first and second mixersrespectively; and

b. means for gating the first and second gated amplifiers.

3. A monopulse-ranging system according to claim 2 which furthercomprises:

a. first and second RF gates;

b. first and second directional couplers fed respectively by the firstand second RF gates, the RF gate-directional coupler combinations beinginterposed one each between the signal-receiving means and the gatedamplifiers; and

c. a calibration generator having an external frequency input source,the output of the calibration generator being fed to the first andsecond RF gates and the first and second directional couplers.

1. A monopuLse-ranging system comprising: a. first and second meansspaced a predetermined distance for receiving a signal; b. a localoscillator; c. a first mixer fed by the local oscillator and the firstsignal-receiving means; d. a variable time delay fed by the first mixer;e. a first envelope detector fed by the variable time delay; f. a secondmixer fed by the local oscillator and the second signal-receiving means;g. a second time delay fed by the second mixer; h. a second envelopedetector; i. a differential amplifier fed by the first and secondenvelope detectors; j. a delay balance discriminator fed by thedifferential amplifier and connected to the variable delay for thecontrol thereof; k. a low pass filter fed by the differential amplifier;and l. means for computing the range fed by the output of the low passfilter, a reference signal from the delay balance discriminator, and thesecond envelope detector.
 2. A monopulse-ranging system according toclaim 1 which further comprises: a. first and second gated amplifiersinterposed between the first and second signal-receiving means and thefirst and second mixers respectively; and b. means for gating the firstand second gated amplifiers.
 3. A monopulse-ranging system according toclaim 2 which further comprises: a. first and second RF gates; b. firstand second directional couplers fed respectively by the first and secondRF gates, the RF gate-directional coupler combinations being interposedone each between the signal-receiving means and the gated amplifiers;and c. a calibration generator having an external frequency inputsource, the output of the calibration generator being fed to the firstand second RF gates and the first and second directional couplers.